The present invention relates to catalytic polymerization of alpha-olefins, and more particularly, to an ultra-high activity non-metallocene pre-catalyst featuring an amine bis(phenolate) ligand-metal chelate and a method for catalytic polymerization of alpha-olefin monomers using this pre-catalyst.
Currently, there is significant interest relating to methods and systems of catalytic polymerization of alpha-olefin monomers based on a `pre-catalyst` featuring a metal bound to one or more spectator ligands, where the pre-catalyst may be soluble in a liquid phase solvent, or is adsorbed on a solid surface, and where alpha-olefin monomer reactant may be liquid or gas phase. In such methods and systems, typically, the pre-catalyst is activated by at least one `co-catalyst`, where the combination of the activated pre-catalyst and the at least one co-catalyst functions as a single chemical entity, or complex `catalyst`, for polymerization of the alpha-olefin monomer. The field of catalytic polymerization of alpha-olefin monomers is of significant industrial importance, as more than 50 million tons of poly(alpha-olefin) products, such as polyetheylenes and polypropylenes, are produced each year, involving metal based catalytic processes and systems.
Hereinafter, the term `pre-catalyst` refers to a chemical entity, in general, and to a chemical compound, in particular, which, when activated by at least one `co-catalyst`, becomes part of a `catalyst` functional for catalytic polymerization of an alpha-olefin monomer, under proper polymerization reaction conditions. Without the presence of at least one co-catalyst, a pre-catalyst is ineffective for catalytic polymerization of an alpha-olefin monomer, and consequently exhibits essentially no catalytic activity for polymerization of an alpha-olefin monomer. Here, when referring to catalytic activity during a polymerization reaction, reference is with respect to the catalytic activity of a pre-catalyst, and it is to be understood that the pre-catalyst is functioning in concert with at least one co-catalyst for effecting catalytic polymerization of an alpha-olefin monomer. Thus, the present invention focuses on a new and novel pre-catalyst compared to pre-catalysts currently used for catalytic polymerization of alpha-olefin monomers.
Currently, one of the major goals in this field is to produce a variety of new types of poly(alpha-olefin) products, for example, polymers made from alpha-olefin monomers featuring more than three carbon atoms such as 1-hexene, having well defined physicochemical properties and characteristics, such as mechanical strength, elasticity, melting point, and chemical resistance, applicable for manufacturing a diversity of end products. This may be achieved by polymerizing different types of alpha-olefin monomers, in order to produce a variety of homo-polymers and co-polymers, with varying degrees of monomer incorporation.
Typically, degree of monomer incorporation strongly depends upon catalyst activity for polymerization of a given alpha-olefin monomer. Recently, Britovsek, G. J. P., et al., in "The Search For New-Generation Olefin Polymerization Catalysts: Life Beyond Metallocenes", Angew. Chem. Int. Ed. Engl. 38, 428-447, 1999, provided a practical quantitative ranking of catalytic activity, with respect to weight of a pre-catalyst, (grams polymer produced)/(mmole-pre-cat. hr), for ethylene polymerization, under one bar pressure, as follows: very low&lt;1, low 1-10, moderate 10-100, high 100-1000, very high&gt;1000. Their ranking is derived from data of catalytic polymerization of ethylene, which is the easiest alpha-olefin monomer to polymerize. Catalytic activity for polymerization of other larger alpha-olefin monomers, such as 1-hexene and 1-octene, is usually at least one order of magnitude less. Thus, a pre-catalyst for polymerization of 1-hexene, for example, may be considered exhibiting high, and very high, activity in the range of about 10-100, and 100-1000, grams/(mmole-pre-cat. hr), respectively.
Physicochemical properties of polymers include, are directly related to, and are controllable by, polymer molecular weight and molecular weight distribution. These values are highly relevant with respect to producing different types of polymers. For example, ultra-high molecular weight polyethylene (UHMWPE), having an average molecular weight above 3,000,000 has the highest abrasion resistance of thermoplastics and a low coefficient of friction. Unlike synthesis of small molecules, however, polymerization reactions involve random events characterized by formation of polymers having a range of molecular weights, rather than a single molecular weight. Typically, polymers are better defined and characterized in relation to narrow molecular weight ranges.
The accepted parameter for defining polymer molecular weight distribution is the polydispersity index (PDI), which is the weight average molecular weight, M.sub.w, divided by the number average molecular weight, M.sub.n, or, M.sub.w /M.sub.n. Depending upon the actual application, ideally, a catalytic polymerization system features `living` polymerization in which the rate of initiation is higher than the rate of propagation leading to a PDI of close to 1, and involving a single catalytic active site. This has been achieved in very few systems for catalytic polymerization of alpha-olefin monomers. A PDI of 2.0, signifying `non-living` polymerization, is often found in metallocene catalytic systems, also involving a single catalytic active site. Classical heterogeneous Ziegler-Natta catalytic systems usually lead to a broader range of molecular weights with a PDI of about 5. One current challenge is to design alpha-olefin polymerization pre-catalysts, and catalytic systems including such pre-catalysts, leading to poly(alpha-olefin) products with low values of PDI.
Metallocene pre-catalysts, featuring a metal complex including a metal atom, for example from Group IV transition elements such as titanium, zirconium, and hafnium, bound to two ligands from the well known cyclopentadienyl (Cp) family of ligands such as pentamethylcyclopentadienyl, indenyl, or fluorenyl, were introduced during the last two decades for the purpose of catalytic polymerization of alpha-olefin monomers. The most common type of metallocene pre-catalyst is a neutral complex including a metal in oxidation state of +4, bound to two anionic ligands in addition to two standard Cp ligands, for example, bis(cyclopentadienyl)titanium dichloride, also known as titanocene dichloride. A related group of complexes is `constrained geometry` pre-catalysts, featuring a metal bound to both a single Cp type ligand and a second anionic group, where the Cp ligand and second anionic group are covalently linked.
Using metallocene and metallocene type pre-catalysts in catalytic processes and systems for polymerization of alpha-olefin monomers affords better control of molecular weight and narrower molecular weight distribution, translating to lower values of PDI, relative to the classical Ziegler-Natta family of pre-catalysts such as titanium trichloride using a trialkyl-aluminum co-catalyst. Metallocene and metallocene type pre-catalysts, processes, and systems are well known and taught about in the art. These pre-catalysts, processes and systems are, however, limited in many respects relating to the above discussion.
Foremost, with respect to catalytic activity, metallocene type pre-catalysts typically exhibit relatively moderate activity for polymerizing a small variety of alpha-olefin monomers. With respect to poly(alpha-olefin) product types and variety, alpha-olefin monomers polymerized by metallocene pre-catalysts are mostly short chain ethylene and propylene, which are already well taught about. Metallocene pre-catalysts are limited in terms of availability and versatility. With respect to pre-catalyst availability, metallocene type pre-catalysts are relatively difficult to synthesize, a fact which affects the possibility of developing new varieties of metallocene type alpha-olefin polymerization pre-catalysts.
Due to continued searching for new poly(alpha-olefin) products exhibiting selected well defined physicochemical properties and characteristics, combined with the above limitations associated with metallocene pre-catalysts, there is growing interest in developing non-metallocene alpha-olefin polymerization pre-catalysts, and related catalytic processes, and systems. The main emphasis is on obtaining new alpha-olefin polymerization pre-catalysts exhibiting higher activity, stability, and availability needed for enabling better control over industrially important polymer parameters such as molecular weight, molecular weight distribution, product type, and variety.
The first step towards development of non-metallocene pre-catalysts was taken by the introduction of a `half sandwich` pre-catalyst, featuring a complex including a Cp type ligand bridging to a heteroatom donor. An example of such a pre-catalyst is a phenolate constrained geometry polymerization pre-catalyst disclosed in U.S. Pat. No. 5,856,258. The pre-catalyst described therein shows relatively high activity of about 1,300 grams/(mmole-pre-cat. hr) for polymerization of alpha-olefin monomers, however monomers polymerized are limited to ethylene, propylene, and styrene.
A non-metallocene alpha-olefin polymerization catalytic system is disclosed in U.S. Pat. No. 5,852,146, and features a bis(hydroxy aromatic nitrogen ligand) transition metal pre-catalyst, functioning with an activating methylaminoxane (MAO) co-catalyst. Relatively high catalytic activity of about 4,000 grams/(mmole-pre-cat. hr) is reported for polymerization of ethylene only. Moreover, MAO is needed in large quantities as co-catalyst, which, in general poses notable limitations relating to cost and containment. MAO used in large quantities is costly, and needs to be properly disposed of with regard to environmental considerations.
Living polymerization of 1-hexene is recently described by Schrock, R. R., in J. Am. Chem. Soc. 119, 3830, 1997, and is disclosed in U.S. Pat. No. 5,889,128. One of the non-metallocene pre-catalyst compositions described therein comprises a dimethyl complex in which the metal atom is chelated to a tridentate spectator ligand, which is activated by a non-MAO boron salt co-catalyst. Catalytic activity under the conditions described was considered high, of about 200 grams/(mmole-pre-cat. hr), and the molecular weight of the obtained poly(1-hexene) product is moderate, of about 50,000 grams/mole.
Living polymerization of 1-hexene is also described by McConville, D. H., in J. Am. Chem. Soc. 118, 10008, 1996. They describe a moderately active non-metallocene polymerization pre-catalyst, exhibiting activity of about 40 g/(mmole-pre-cat. hr), involving activation of a pre-catalyst featuring a dimethyl metal complex of a bis(amide) ligand, with a non-MAO boron Lewis acid as co-catalyst under room temperature, for producing a moderate molecular weight polymer, of molecular weight of 40,000 grams/mole. The same pre-catalyst, but functioning with MAO as co-catalyst in large excess, under the same reaction conditions, yields significantly higher activity, as reported by McConville, D. H., in Macromolecules V. 29, 5241, 1996. Again, limitations associated with using MAO as co-catalyst are present.
Another active non-metallocene living 1-hexene polymerization pre-catalyst functioning with a non-MAO co-catalyst, is reported by Kim, K., in Organometallics 17, 3161, 1998. The described catalyst system exhibits activity of about 400 gramsl(mmole-pre-cat. hr).
A non-metallocene non-living 1-hexene polymerization pre-catalyst is disclosed in U.S. Pat. No. 5,807,801. The pre-catalyst exhibits high activity, of on the order of 10.sup.6 g/(mmole-pre-cat. hr), when the pre-catalyst is, again, activated with MAO as co-catalyst, for the polymerization process taking place at 50.degree. C.
A non-metallocene bis(phenolate) pre-catalyst is reported by Schaverien, C. J., in J. Am. Chem. Soc. 117 3008, 1995. The pre-catalyst described exhibits limited activity, of about 10 g/(mmole-pre-cat. hr), for tactic polymerization of 1 -hexene, yielding high molecular weight isotactic poly(1-hexene).
In view of the above discussed limitations, to one of ordinary skill in the art, there is thus a need for, and it would be advantageous to have an ultra-high activity non-metallocene pre-catalyst and corresponding method for catalytic polymerization of alpha-olefin monomers, not limited to activation by large quantities of a co-catalyst such as MAO, and also characterized by high stability, readily obtained or synthesized, and capable of producing different types and varieties of poly(alpha-olefin) products having high molecular weight, and low molecular weight distribution. Moreover, there is a need of such a pre-catalyst and method for producing alpha-olefin polymers other than polyethylenes and polypropylenes, having industrially applicable properties and characteristics.